Asphalt is the name used in the UK and Europe to denote the material, used in road building and other civil engineering applications, which comprises aggregates (e.g. crushed rock, gravel, shingle, sand and recycled broken up asphaltic road surface material) coated in bitumen. In the USA, this material is generally known as asphalt concrete.
The aggregates used in making asphalt typically contain substantial quantities of water, either because of the wet nature of the medium from which they have been extracted, or because they have been left out in the open and have therefore been exposed to atmospheric moisture. Consequently, the aggregates need to be dried before use. Moreover, in order to ensure efficient mixing of the aggregates and bitumen and maximise the binding of the bitumen to the aggregates, it is desirable that the aggregates should be heated prior to mixing with the bitumen. For these reasons, the aggregates used in making asphalt tend to be heated to temperatures in the range from 150 to 190° C. or higher. In some asphalt mixes such as hot rolled asphalt (HRA), temperatures as high as 220° C. to 230° C. are used.
A typical asphalt plant will therefore comprise a dryer for drying and heating the aggregates. A common form of dryer used in asphalt plants is a rotating drum dryer in which the heat for the drying process is provided by one or more combustion burners at one end of the drum. Air is drawn through the combustion burners and the heated gases from the burner pass along the interior of the rotating drum and out through a gas exhaust outlet at the far end of the drum. The stream of hot gases from the burner passing through the drum serves to dry the aggregates. In order to facilitate the drying and heating process, the internal side wall of the drying zone of the drum is provided with a series of scoops or blades which scoop up the aggregates from the floor of the drum, lift them to the high point of revolution of the drum and then drop them so that they fall back as a curtain of aggregates through the stream of hot gases to the floor of the drum. In most known types of drum dryer, a contra-flow arrangement is used in which the drum is inclined so that the drying aggregates gradually migrate from an inlet at the end of the drum opposite the combustion burner towards the end at which the burner is located. Once they have reached the burner end, the dried hot aggregates are discharged into a conveyor device, such as a bucket lift, which carries them to hot aggregate storage containers where they are stored prior to be mixed with hot bitumen to form asphalt.
A problem with many known dryers is that, for a variety of reasons, it has proved difficult to achieve efficient combustion. The short combustion zones found in many dryers and the interference caused by falling aggregates and airborne dust each contribute to inefficient combustion.
If combustion of the fuel is incomplete, the exhaust emissions vented into the atmosphere will contain large concentrations of pollutants such as carbon monoxide, sulphur and nitrogen oxides. In addition to being atmospheric pollutants, unburnt fuels will condense on the bag filters used in many aggregate drying plants shortening the useful life of the filters thereby increasing costs in replacement bags.
In order to prevent incomplete combustion, it is customary to set up a burner so that an amount of air in excess of the stoichiometric amount required for combustion is mixed with the fuel prior to or during combustion. However, if too much excess air is introduced into the burner, this will lead to inefficiency and in particular inefficient transfer of heat between the air passing through the drying drum and the aggregates in the drying drum. Ideally, therefore, a burner should be set up to use the lowest possible level of excess air needed for complete combustion.
Hitherto, it has proved very difficult to control the amount of excess air used in asphalt plant dryers and, typically, asphalt plant dryers tend to be “over-aired”. Thus, whereas the gas burners used in boilers may function well using as little as 5% excess air, asphalt dryer plants typically operate with excess air levels anywhere between 100% and 1000%. As a consequence, asphalt dryer plants are typically very inefficient in terms of their energy consumption.